A.-c. motor control system



Oct. 25, 1966 c. c. ROE

A.C. MOTOR CONTROL SYSTEM 5 Sheets-Sheet 1 Original Filed July 6.` 1962 Cel. 25, 1966 c. c. ROE 3,280,928

A.C. MOTOR CONTROL SYSTEM Original Filed July 6, 1962 5 Sheets-Sheet 2 oct. z5, 1966 C. C. RQE 3,280,928

A.C. MOTOR CONTROL SYSTEM Original Filed July 6, 1962 5 Sheets-Sheet 5 Charles C. Roe, deceased,

United States Patent O 11 claims. (Cl. 18o-6.5)

This is a division of copending lapplication Serial No. 207,886, tiled July 6, 1962, and assigned to the assignee of the present application.

The presentinvention relates to control of A.C. motors `and more particularly to means for controlling the speed and torque thereof and finding particular application in an e-arth moving vehicle but which is, nevertheless, useful wherever speed or torque variation under m-anual or automatic control is desired.

It is an object of the present invention to provide an A.C. motor control system which 4avoids the expense and complexi-ty usually associated with A.C. control but which has control characteristics which are equal to and in many respects better than those of a highly developed D.C. motor control system. More specifically, it is an object to provide an A.C. motor control system which employs an A.C. motor of the type having two relatively adjustable stat-ors cooperating with a single rotor in which the motor characteristics .are achieved by varying the physical phase position of the two stators relative to one another, but which incorporates Ia number of important improvements which overcome the disadvantages which have over the years been associated with this general type of motor.

It is another object of the present invention to provide an A.C. motor control system for use with dual-stator motors which requires only small control forces to cover the complete range of speed and torque from zero to maximum and which permits such control to be .achieved with small displacements, thereby permitting use of conventional small-stroke pneumatic and hydraulic as well as direct mechanical controls. More specifically, it is an object to provide an A.C. motor and contr-ol arrangement in which control may be effected by s-imple pos-itioning of a control element without necessity for overcoming any of the torque reaction of the motor and which, therefore, permits small components to be employed with motors having a wide range of size up to many hundreds of horsepower. yIt is also an object in this connection to provide an A.C. motor control system which is ideally suited for remote control from a control station wi-th simple control linkage suiicing between the control station and the motor in lthe form of mechanical, pneumatic or hydraulic connections.

In one of the aspects of the present invention it is an object to provide a motor control arrangement incorporating means for automatic reversal at the center of the control range so that a single controller suffices to cover the complete range of torque and speed available in both directions. In this connection, it is an object to provide an A.C. motor control utilizing a single control element for forward and reverse operation and which may be moved from forward to reverse or vice versa to elfect dynamic braking and which, therefore, avoids the necessity for conventional brakes to bring the load to a stop, or to effect reversal, in the shortest possible time.

It is another object of the invention to provide in a system of the above type means for automatically limiting to a predetermined safe value the torque which may be exerted by the motor, and hence the maximum current which can be drawn by the motor under conditions of acceleration or braking. Hence, it is an object to provide an A.C. motor control system which is safe and which may be placed in the hands of an inexperienced operator without risk of damage due to overloading or other careless operation. Since only one control element need be used to obtain the complete range of speed and torque of a motor in both directions, instructions to the operator may be minimized.

It is an important object of the present invention to provide a motor control system which is ideally suited for the -driving of an earth moving vehicle utilizing plural driving motors supplied from a common prime mover and particularly where individual motors are associated with the individual driving wheels. Consequently, it is an object to provide an earth mover drive utilizing alternating current which permits simpler control ofthe speed and -torque of the driving wheels but which includes novel provision for insuring that full torque is applied to the two wheels. Thus, the control system is well suited for diving a conventionally steered vehicle with equal torque being applied to the driving wheels on the 4opposite sides of the vehicle without requiring any special attention on the part of the operator.

While the device has certain novel features and advantages when applied to conventionally steered vehicles, it has particular utility in skid steered vehicles of the type where the supporting wheels are mounted for rotation about ixed axles and where turning is achieved by driving the wheels, or tracks, on the opposite sides of the vehicle at dilerent speeds. It is a more specific object to provide an A.C. driving arrangement for a vehicle of the skid steered type which avoids the necessity for the large and expensive brakes usually required to hold a wheel on one side of the vehicle stationary while the wheel on the opposite side rotates. On the contrary, the present invention permits the wheels to be rotated in opposite directions to permit a radius of turn which is extremely short and which, as a limiting case, enables pivoting of the vehicle about a vertical axis without any forward motion. Consequently, it is an object to provide a driving and con-trol arrangement for an earth moving vehicle which permits a exibility of operation heretofore not available in vehicles of this ltype and which brings about such operation by control means Wh-ich are so simple that the opera-tor of the machine c-auses the machine to do his bidding by reflex action and without conscious effort and so that attention may be directed toward planning ahead the movements of the vehicle for maximum safety and etiiciency.

While the invent-ion is particularly well suited for use in an earth moving vehicle, its inherent advantages cause it to be similarly well suited for other uses employing either single drive motors or plural, cooperating drive motors wherever a ine degree of control is required as,

essing equipment, conveyors, hoi'sts, Winches or wherever speed or torque must be varied. Or, if desired, motors and control systems of the present type may be used in applications where conventional induction motors normally suffice but where versatili-ty of the driven machine is increased by the present control feature.

It is a related object to provide an A.C. drive and control system which is lwell suited to applications formerly requiring elaborate arrangements to produce a desired output characteristic. For example, the present motor may be employed for awinch for handlingrefueling lines between ships or other applications where the motor must be capable of rotating in either o-ne direction or the other for take up or pay out or remain stationary while maintaining the torque within a desired range. It is a more specific object of the present inventionto providey a control for an A.C'. motor in which the range of speed and torque is coveredl by the physical positioning of ay control. element andi in which thel electrical switching required for reversal is. achieved Without necessity for interrupting large amounts of current. On the contrary, the electrical switching. is caused to occur under conditions of Zero voltage and zero current automatically and without care or attention on the part of the operator thereby enabling use. of low power switches andv contacters.

It is asgeneralobjec-t to' provide an A.-C. motor and control system in which the motor may be produced at a cost which does not appreciably exceed the cost ot a conventional rnotor of the `same maximum power rating and in which the auxiliary equipment required for use with the motor costs only a. small fraction of that required in employing conventional motors in either an A.C. or D.C.

system. Y

It is another general object to provide an motor control system whichy has high efficiency and in which the load current under the various possible co'nditions of operationincluding the stalled condi-tionwhen the system is acting as a torque motor, is minimized'. It is an object of the invention in one of its aspects to provide an A.C. motor control system employing a dual stator and wound rotor which insures even distribution of current ow in the rotor windings or bars and Iwhich therefore avoids the concentration of current which may exist in the rotor lbars of conventional two-stator machines when the stators are ofs'et from one another. In this. connection it is an object to provi-de an A.C`. motor havingy a' wound rotor but which may be employed, under 'blocked rotor conditions, as a variable transformer in which adjustment of voltage may be brought about using low power control elements. In the present construction, employing an equalizing linkage between thev two stators, the control need not overcome the reaction torce but need have power sufficient only to. overcome the friction in moving the controlled stators between successive positions ot adjustment.

Notwithstanding all of the above advantages it is an object to provide an A.C. motor control system which is Ysimple and inexpensive, which is free of reliance upon electronic or other delicate control; components, andv which may be serviced. or maintained in the field by one having only a rudimentary knowledge of A.C. motor andv control devices.

Other objects and' advantages of the invention will becorne apparent upon reference to' the detailed description and attached' drawings in which FIGURE 1 is a side elevation of an A.C. motor' embodying a simplified form of the present invention.

FIG. 2 is a transverse section taken through the motor along line 2-2 in FIG. 1.

FIG. 3` is a longitudinal section taken along the line 3-3 in FIG. r2.

FIG. 4- is a fragmentary sectionl taken along the line 4"-4 in FIG. 1 showingI the mounting of the differential linkage. n

FIG. 5 is a fragment showingv the nature of the connection between one end of the linkage and the associated stator.

FIG. 6 shows the supply wiring and reversing contactor for energizing the motor of FIG. l.

FIG. 7 is a fragmentary somewhat diagrammatic view of an alternate mounting for the reversing switch -between the adjacent stators.

FIG. `8 shows a system o'f the present type incorporating a yielding connection for automatic torque control.

FIG. 9 is a diagram showing the differential linkage of FIG. 8 in three positions in order to bring out the sequence of operation.

FIG. 10 shows motors of the abovel disclosed type employed as part of a novel driving land control system in a conventionally steerable vehicle.

FIG. l1 is a plan view on a small scale steerable vehicle.

FIG. 12 illustrates a skid-steered tractor employing a drive system in accordance with the present invention.

FIG. 1-3 is a plan view on a small scale of the tractor of FIG. 12'.

FIG. 1:4 is ai diagram showing. the control system ernployed in the tractor ot lFIG. 1-2.

FIG. l5 shows a family of speed torque curves. obtainable at `successive settings of the dilerential linkage together with representative load curves.

FIG. 16 is a diagram showing the inventive structure employed, with blocked rotor, as -a voltage regulator.

While the invention has been described in connection with certain preferred embodiments, the invention is not limited' thereto and i-t is intended to cover the various alternative and equivalent constructions included within the spirit and scope of the `appended claims.

Turning now to FIGS. 1*-5 of the drawings, there is disclosed a motor 20 having a frame which consists of end members 21, 22 and a cylindrical housing 23. Extending axially thro'ugh the housing is a rotor shaft 2S mounted in suitable bearings 2'6. Securedto the shaft is a rotor assembly 30 which', in the present instance, consists of two portions 31, 3'2. Engaging both portions of the rotor is a winding 33 which is brought -out to' sliprings 3'4, 35, 365. Spacedf sidel by` side in the frame and alined with the portions 31, '32 of the rotor arel stator assemblies 41, 42 having windings 43, I44, respectively. The windings 43; 44 are preferably three phase windings 'which may either-be lapA woundor' wave wound having connections 45, 46, respectively, for connection to the regula-r three phase supply line. In accordance with well known A.C. motor practice, the windings mayl be designed for fourpole opera-tion to produce a nominal speed of 175'()y rpm'.

A.C. motors are known in the art having separate stators arranged side by side and cooperating with a single roto-r assembly and with one of the stators being angular- 1y adjustablel in order to adjust its physical phase positioning with respect to theI other thereby tovary the motor output characteristics. Thus, in prior `art devices one of the stat'orsmay be adjusted to a null position inwhich the voltage which it induces iny the rotorl winding is oppositely phased?, and hence cancels, the voltage which is induced by the second stator so that the torque applied to the rotor is zero. Alternatively the movable stator may be adjusted to a position in which the induced voltages are additive for production of' maximum torque.

In accordance with the present invention both of the stators 41', 42: are made rotatable with respect to the motor frame and are interconnectedy by a dilierential linkage having its endsconnected to the stators and which has its neutral point anchored with respect to the motor frame soL that the twoL stators are cons-trained to move equally and in opposite directions from the null position Iupon positioning by a control member. In the embodiment of the invention illustrated in FIGS. 1'5, the differential linkage, indicated generally at 50, is in the form of a centrally pivoted lever havingv a first end-y 51 which is connected to the stato-r 41 andy a second endY 52 which is-.connected to the stator 42. The

showing the linkage is centrally pivoted on the motor frame by a bolt or the like 53. For the purpose of coupling the stators to the ends of the differential linkage and to accommodate the relative endwise movement which occurs when the lever is rocked from the neutral position, telescoping ball-and-socket connections are provided. Thus, arms 61, 62 extend radially outward from the stators 41, 42 carrying balls 63, 64, respectively (FIG. 4), each having a central bore. The ends of the lever carry cylindrical pins 65, 66 which are telescoped into the balls. Slots 67, 68 are formed in the motor housing to provide clearance for t-he arms 61, 62.

For the purpose of minimizing friction between the stators 41, 42 and the motor frame, bearing-s are interposed which, in the present instance, consist of double rows of ball bearings 69 occupying suitable complementary grooves formed on the outside of the stator assemblies and the inside wall of the housing 23. It will be apparent, however, that other anti-friction bearing arrangements may be employed between the stators and the frame without departing from the invention.

In order to permit the ends of the lever to follow the curvature of the motor frame, the lever is provided with transverse hinge connections 51a, 52a.

In carrying out the present invention, the stators 41, 42 are so phased with respect to one another that when the arms 61, 62 are in alinement, Wit-h the differential lever 50 horizontal, the stators define a null condition in which the induced voltages are opposite in phase and cancel one another. Manually controlled means are provided for tilting the lever Stl so that the stators are adjustably offset with respect to one another thereby to produce a net induced voltage in the rotor winding. In the simplest form of the invention such positioning means includes a quadrant lever 70 which is co-nnected tothe lever 50 by means of a link 71 having pivot connections '72, 73 at its ends.

In accordance with one of the aspects of the invention, means are provided for reversing the connection to the stator windings as the quadrant lever 79 transverses the null position so that forward rotation of the motor is obtained when the quadrant lever is moved in one direction from neutral and reverse operation is obtained when the quadrant lever is moved in the opposite direction. In order to understand the means employed for bringing about the reversal, specific reference is made to FIGS. 1 and 6. Thus, interposed between the three phase supply line and the leads 45, 46 which are connected to t-he stators, is a reversing contactor 7S which is under the control of a switch 76 having a plunger s77. Means are provided for causing operation of the switch 76, and hence of the reversing contactor 75, at the null whenever the quadrant lever 76B is rocked in the reverse direction. In the present instance this may be simply accomplished by forming a cam rise 7S on the link '71 which is coupled to the quadrant lever. Or, if desired, the same effect may be achieved by employing a reversing switch which is directly coupled to the two stators and which is operated when the stators are moved relative to one another in one of the two possible directions from the null, namely, the direction which corresponds to the reverse setting of the quadrant lever. Thus, as shown in FIG. 7 a reversing switch '76a is provided having a plunger 77a, the body of the switch being mounted upon the stator 42 and the plunger 77a cooperating with a cani rise 78a on the stator 41.

In operation and with the quadrant lever set in neutral position, the stators 41, 42 are in their null position so that the motor does not rotate even though the motor leads 45, 46 are energized. When rotation of the motor is desired in the forward direction, the quadrant lever is moved forward, progressively tilting the differential lever 50 so that the stator 41 is rotated clockwise and the stator 42 counterclockwise as viewed from the right hand end of the machine. As the two stators are thus moved 6 into phase alinement, the voltages which they induce in' the rotor are progressively in phase producing progressively increased torque. In the case of a four pole motor, full torque is produced when the two stators are offset with respect to one another by ninety degrees as shown by the dotted outline in FIG. 1.

In order to brake the motor to a stop and produce rotation of the shaft in the opposite direction, the quadrant lever is pulled back to its neutral position. If the lever is pulled back rapidly, any tendency for the rotor to overrun results in ,generator action. When the quadrant lever traverses the neutral position, the cam rise 78 engages the plunger 77 of the reversing switch thereby causing the contactor 7S to reverse the connections to the motor leads. Specifically, two of the three leads are switched with respect to the two supply lines which they engage. The result is to produce a field which rotates in the opposite direction. Thus, torque is applied to the rotor in the opposite direction which brakes the load to a stop and then causes the load to rotate in the opposite direction. When the end point of the stroke is reached, indicated by the dot-dash outline in FIG. l, the reverse torque is maximum. The quadrant lever may then be restored to its neutral position and may be momentarily moved beyond neutral into the forward range to effect braking.

While mention has been made in the above description of increasing and decreasing the motor torque, it will be understood that this refers to torque capability with the motor connected to a load. Also it should be kept in mind that while the above description has mentioned variation in torque from Zero to maximum in each direction, the control, in a practical case, permits the speed to be varied over a wide range depending upon the speed-torque characteristic of the load and the speedtorque characteristic which is applicable to the motor for any given setting of the control member. Briey stated, the speed at any setting of the control member is that which will cause the torque exerted by the motor to equal the torque reaction exerted by the load so that a condition of equilibrium is established at which the motor speed tends to remain constant.

With the operation of the motor in mind, more specific reference may be made to the nature of the rotor winding 33. Preferably the winding 33 is of the wound type rather than the squirrel cage type. A wound winding has a number of advantages in the present combination. In the first place the heating effect of the rotor current is more evenly distributed throughout the rotor. Next it permits the ends of the windings to be conveniently brought out for the connection of auxiliary or series resistors, shown at 81, 82, 83, (FIG. 1) for adjustment of the speed torque characteristics and, more particularly, for enabling any desired torque to be achieved under starting conditions. That is to say, under starting conditions, and when the rotor is turning at substantially less than synchronous speed, the auxiliary resistors 81, 82, 83, which are externally mounted, enable high torque and serve to dissipate the energy of slip in the for-m of heat safely outside of the machine. Preferably the resistance is just high enough to produce adequate starting torque for the intended usage. It is found that the inclusion of even a small amount of resistance, in a practical case on the order of a tenth of an ohm or less, causes the reactive ux, due to the current flowing through the rotor winding, to be offset sufficiently with respect to the fiux of the rotating field so that the field ffux is enabled to penetrate the rotor. Where the invention is employed with the rotor of the squirrel cage type, resistance can be incorporated, but varying its value is less convenient. In any event, it will be found desirable to insulate the rotor bars in order to prevent the setting up of circulating currents in the two portions of the rotor, particularly under null conditions.

It is one of the primary advantages of the above depoint of the differential linkage.

scribed motor and control system that speed and torque variationsl may be achieved with only a light control linkage and using small forces exterted over small distances. This is true even though the motor may have a capability of many horsepower. The reason for this is that the control member is not required` to overcome any of the reaction torque of the two stators. Instead, because of the differential nature of the linkage, the reaction torques of the two stators produce torques on the differential lever 50 which are in opposite directions and which, using identical stators, precisely cancel one another, while the total reaction torque is applied to the motor fra-me at the central point of pivoting 53. The practical result of this is that the differential lever may be adjusted to a given angled position by only light ngertip pressure, the amount of pressure being independent of the torque being exterted by the motor. Normal friction is sufficient to maintain the control member in a set position, although, if desired, additionalfrictionmay be incorporated as, for example, by tightening the point of pivoting of the quadrant lever 70.

While the invention hasV been described in connection with a particular form of differential linkage, namely, a centrally pivoted lever for the sake of simplicity, it will be understood by one skilled in the art that the invention is not limited to use of a differential lever but contemplates the use of any differential connection, the result of which is to produce equal and opposite movement of the two stators relative to the frame in which they are mounted. Thus, if desired, other specific lever arrangements may be employed or the two rotors may be coupled together by differential gearing or by a differential cable arrangement, or the like without departing from the invention, the neutral point of the linkage coupled to the motor frame or to the structure to which the motor frame is secured.

In accordance wit-h one of the aspects of the present invention, it is proposed that yield-ing means suc-h as a spring or preloaded actuator be interposed between the neutral point of the linkage and the frame of the machine in place of the rigid connection 53 described above, thereby to limit the torque which may be exerted by the motor for any position of adjustment of the control member. More specically, in accordance with the present invention, the reaction torque which is exerted by the stators is caused to bring about a corrective and automatic variation in torque to reduce the torque to predetermined level. Such a torque limiting characteristic is desirable for a number of applications, for example in the operation of a winch used to pay in or payy out cable between ships travelling side by side in a refueling operation, reference being made to FIG. 8. While this gure is diagrammatic, it will be understood that the motor shown therein corresponds to that previously described except for the mounting of the neutral Thus, a motor 120 is provided having a shaft 125 carrying a suitable rotor assembly cooperatingl with stators 141, 142, respectively.

interconnecting the stators is a differential linkage or lever 150 having ends 151, 152. In place of the nonyielding pivot connection 53 at the center of the lever, the pivot connection is provided with springs 154, 155 which are seated with respect to the motor frame at 156, 157.

For the purpose of adjusting the nominal position of the differential lever, a quadrant lever 170 is provided which is coupled to the lever 150` by a link 171 having pivots 172, 173 at its ends. In. order to maintain the set position of the quadrant lever 170, means (not shown) are preferably provided for inducing a certain amount of friction for example at the quadrant lever pivot. Connected to the shaft 125 of the motor is a stepdown drive connection 160 which should preferably be of the reversible type where use is to be made of the motors constant torque characteristics. By the term reversible is meant that the drive connection is capable of power flow in either direction which is to be distinguished from the usual worm gearing in which power can be transmitted only in one direction.

The operation of the device to produce torque limitation at the connected winch will be apparent upon reference to FIG. 9. The differential lever normally, under the olf condition, occupies the horizontal position indicated at A. When the quadrant lever 170 is moved from neutral the differential lever 150 tends to move to a nominal anfvled position as indicated at B which corresponds to a certain torque output capability. However, when torque is exerted against the load, the total stator reaction torque is applied to the springs so that spring 154 tends to be compressed, as shown, and the spring 155 tends to be elongated. This causes the center of the lever 150 to move from the point 153 to the point indicated at 153:1. Assuming that the quadrant lever has sufficient friction to hold it and its link 171 in the set position, the pivot 172 at the inner end of the link 170 will tend to remain stationary so as to become the fulcrum for any further lever movement. As a result, shifting of the center from the point 153 to the point 153a is accompanied by a change in the angle of the differential lever. Thus, the differential lever tends to occupy the final position C at a relatively shallow angle, thereby limiting the torque and tailoring it to the function to be performed.

The automatic self-corrective action of the above driving arrangement will be apparent by considering the effect of an increase or decrease in the torque exerted by the load. Assuming that the load is in the form of a cable interconnecting two ships in the event the ships move closer together, the reduction in the force exerted by the cable produces a reduction in load torque and hence a reduction in the reactive force exerted against the spring 154. The change in spring dimension (elongation) causes a shift of the fulcrum point, downwardly in FIG. 9, increasing the angling of the differential lever thereby shifting the operating point on the speed-torque characteristic of the motor, causing the motor torque to overcome the reaction torque of the load to wind in the slack cable. When the end of the slack is reached, the reaction torque of the load increases thereby compressing (shortening) the spring 154 so that the center of the differential lever again tends to move toward pivot position 153:1, causing the differential lever to occupy a shallower angle and reducing the motor torque so that no further cable is woundk in. In the event that the ships tend to separate, causing increased tension on the cableffurther compression of the spring 154 occurs, causing the differential lever to occupy a still shallower position and momentarily reducing the torque so that cable may be played out from the winch. The net result is to provide an automatic winch control which tends to maintain a predetermined tension in the cable independently of whether cable is wound in or played out and with the amount of tension being adjustable simply by setting the quadrant lever 170.

In place of the springs 154, 155, a uid actuator may be used supplied with lluid at a regulated pressure for yielding when a predetermined torque is exceeded.

It will be apparent to one skilled in the art that the present motor control system is ideally suited to perform the function of a torque motor with the rotor operating Iunder stalled conditions and with rotation in one direction or the other only to the extent necessary to maintain torque equilibrium. It is found that the present motor is ideally suited to such operation since the net voltage which is induced in the rotor winding and which sets up circulating current therein is only that which is required to exert the desired torque and below the level which would cause overheating. It will also be apparent that the arrangement disclosed in FIG. 8 is not limited to operation in the vicinity of a stall but permits operation over a wide range of speed and torque where coupled to other types of loads. Where desired to operate the motor of FIG. 8 for driving in both directions, the reversing switch and circuitry of FIGS. 6 and 7 may be readily incorporated.

If desired, the springs 154, 155 may be utilized simply for protective purposes, by proper choice of prestress and spring rate so that yielding occurs only when the torque loading exceeds safe limits. Thus, the present type of drive may be placed in the hands of a relatively unskilled operator with assurance that the motor will not be burned out or overloaded by mishandling of the controls.

The present invention is also uniquely applicable to the driving of vehicles of both the conventionally steered and the skid-steered type. Application of the invention to a conventionally steered vehicle is shown more or less diagrammatically in FIG. 10, with the vehicle itself being shown in plan view and at reduced scale in FIG. 1l. Here the vehicle, indicated at 200, has driving wheels 201, 202 and steering wheels 203, 204. The vehicle has a prime mover in the form of a gasoline or diesel engine 205 coupled to a three phase A.C. generator 206 which supplies current to two A.C. motors 220, 220a of the type described above.

Turning to FIG. 10, the drive motor 220, by way of example, has stators 241, 242 cooperating with a rotor (not shown) having a shaft 225. The shaft is connected to the driving Wheel 201 via a suitable stepdowu connection 230 which may, for example, be a set of planetary gearing. Similarly, the right hand drive wheel 202 is driven by the motor 220a having stators 241er, 242:1.

In carrying out the present invention, means are provided for manually and simultaneously setting the differential linkages 250, 250a of both of the motors. This is accomplished in the present instance by a control 260 controlling positioning servos 261, 262, respectively, via air or hydraulic lines 263, 264. Air or hydraulic uid is supplied to the control 260 via a supply line 265 which is connected to a suitable pressure source. The positioning servo 261 is shown in simplified form consisting of a conventional double ended actuator having a plunger 266 and a centering spring 267. The companion servo has a plunger 268 and centering spring 269.

The operation of the control 260 is such that when the quadrant lever, indicated at 270, occupies its central or neutral position, the pressure in the output lines 263, 264 is zero so that the plungers 266, 268 cause the differential levers 250, 250a to occupy the horizontal position as shown. In lieu of a double acting pressure regulator, a conventional single acting regulator 260a may be adapted to the present use, the output feeding a selector valve 269]) which is thrown to either a first position to apply the output pressure to line 263 or a second position to apply the output pressure to line 264. Throw-over of the valve takes place during the initial movement of the quadrant lever in either direction. The further movement of the quadrant lever is accommodated by the takeup springs 260C.

For the purpose of opposing the reaction torque of the stators of each of the motors, the neutral points of the differential levers are connected to individual actuators capable of yielding when a predetermined torque is exceeded. These actuators, indicated at 271, 272 are connected to the air supply by means of an adjustable reguiator 273 which determines the yield point. For the purpose of permitting the actuators 271, 272 to operate in either direction, a selector valve 274 is provided which is under the control of the pilot actuator 274a connected to the pressure lines 263, 264. It will sufice to say that when Iline 263 is active pressure is applied to one side of the actuators 271, 272, whereas, under the opposite condition, when line 264 is active, pressure is applied to the opposite side of the actuators 271, 272.

Prior to describing the operation of the system, mention .may be made of the means for reversing the phase of the current supplied to the two driving motors. This is ac- 10 complished by providing a lreversing contactor 275 which is under the control of a reversing switch 276. A corresponding contactor 275a and switch 276a are used with the opposite driving motor.

In order to understand the features and advantages of the driving arrangement of FIG. 10 it will be assumed that the vehicle is at rest, that the quadrant lever 270 is in neutral position and that the motors are energized with the rotors stationary. It will further be assumed that the regulating valve 273 associated with the torque reaction actuators 271, 272 is adjusted to a point which will limit the torque exerted by the two motors to a sate value. To cause the vehicle to go forward, the operator moves the quadrant lever 270 in the forward direction. This pressurizes the line 263 which causes the servos 261, 262 to move the differential levers 250, 250a to an angled position, similar to the manner which has been described in connection with FIG. 9. Thus, both driving motors produce torque and lthe vehicle proceeds to move forwardly at a lspeed which is dependent upon the setting of the quadrant lever.

In the event of a turn, for example, ra turn to the left, the left h-and driving wheel is required to slow down and, in the case of an extremely sharp turn, the drive wheel and associated motor 220 may have to slow nearly to a stal-led condition. The resulting increase in torque may exceed the safe value; if so, t-he pressure in the actuator 271 is overcome so that yielding movement of the plunger therein occurs to reduce the degree of angling of the differential lever as described in FIG. 9. This reduces the torque to a safe level until the turn has been completed.

It is one of the features of the present driving arrange- -ment that the vehicle can be brought to a stop quickly by means of motor braking. Thus, to stop, all that the operator has to do is to move the quadrant lever 270 back to neutral and thence in the reverse direction. Movement through neutral effects reversal of t-he reversing switches 276, 276a so that torque is subsequently applied to t-he rotor in the opposite direction. Movement through neutral also reverses the direction of applied pressure in the servos 261, 262 and Iin the torque reaction actuators 271, 272. Consequently, with continued movement of the quadrant lever in the reverse direction, torque is applied to the driving wheels in a direction to arrest forward movement and, as t-he vehicle comes to a stop, the quadrant lever is restored to its neutral position. In the event that the quadrant lever is t-hrown so violently in the reverse condition as to run the risk of overloading the motors or generator, yielding takes place at the actuators 271, 272 causing a shift of fu-lcnum of the differential levers 250, 25011 so that the levers assu-me a shal'lower position, with reduced applied torque. This is notwithstanding the fact that the quadrant lever 270 may be in its full reverse position.

It is apparent then that a drive system has been produced for a steerable vehicle which permits controi by a Asingle quadrant Jever in either the forward or reverse directions and which, upon throwing the quadrant lever to its opposite position, results in immediate braking, with the motors and generators being protected in both the driving and braking conditions against flow of excess current. Consequently, a vehicle having the present control system ma'y be operated by la relatively inexperienced operator with instinctive control and Without hazard to the equipment.

In accordance with one of the aspects of the invention there is provided a skid-steered control system for use in tracked vehicles and where the differential movement of the tracks must be relied upon to provide steering in one direction or the other. More specifically, there is provided a driving and contrel system in which the forward and reverse movement of the tracks on the opposite sides of the vehicle, and the degree of braking provided by su-ch tracks is under the control of left and right hand quadrant levers with provision for protecting the right and left hand `drive systems against overload.

Referring to FIGS. 12 and 13, there is shown a tracked vehicle 300 which may, for example, be the Caterpillar model D8 or D9 tractor. The vehicle, has tracks 301, 302 having a prime mover 305 and generator 306. The generator, which is preferably olf the three phase tylpe, supplies current to the motors 320, 320a which are coupled to the tracks by stepdown driving connections 330, 330a which `are perferably in the form of planet-ary gearing. Foc-using attention lupon the left hand driving system, the motor 320 has stators 341, 342 which are coiupled together by a differential lever 350. The position of the differential lever is established by a manually operated double acting pressure regulating valve 360 which controls the position of a servo 361 to which it is connected by supply lines 363, 364. The valve and servo correspond to the units 260, 261 previously described. Air o-r hyd 'aulic fluid is received fro-m a suitable source via a supply line 365. The servo 361 is coupled to the differential lever 350 by means of a plunger 366. The servo includes a centering spring 367 as in the preceding embodiment.

For the purpose of Varying the torque which may be exerted by the motor, and for limiting the torque under extreme conditions, an actuator 371 is provided which is connected to the center of the differential lever 350 and which is alternatively supplied by the pressure lines 363, 364. IReversall of the phase of the current supplied to the motor stator windings is brought about by a reversing contacter 375 which is under the control of a reversing switch 376, the switch 376, by way of example, being secured to the stator 342 a-nd under the control of a cam surface 378 associated with the stator 341.

The system employed to drive the right hand track is identical to that just described and corresponding reference numerals have been employed with the addition of subscript a.

In operation the two quadrant llevers 370, 370a are moved by equal amounts in the forward direction. This Ipressurizes the lines 363, 363a causing the differential levers 350, 350a to be angled substantially in the same amount. Simultaneously pressure is applied via the lines 363, 363g to the torque reaction actuators 371, 371a which `assume the reaction torque so that the vehicle moves ahead. When it is desired to turn, for example to make `a turn to the left, the quadrant lever 370a is moved for Iforward which increases the torque output of the right hand motor resulting in increase in the speed of the right hand track so that the vehicle turns to the left in a wel/1 known skid turn.

In accordance with one of the aspects of the present invention, the inside track, in this case the left hand track, may be caused to come to a stop by moving the quadrant lever 37 0 back through neutral and thence in the reverse direction. Sufficient reverse torque may thus be pro-duced so that the lef-t hand track is held stationary. Upon still further improvement of the quadrant lever 370 in the reverse direction, the left hand track may be caused to reverse its moveme-nt so that the vehicle literally pivots about it-s center. It may benoted that sharp turns and pivot turns Vare achieved usi-ng the present system without the necessity for the usual brakes conventionally required in tractors of the type disclosed. Thus, braking is achieved by moving the quad-rant lever in the opposite direction, and the inside motor, when operating at or near stall, 4simply acts as a t-orque motor as in the case of the winch described above in connection with FIG. 8. The stalled current is, however, kept within safe limits, as described, by reason of the yielding which occurs at the torque reaction actuator, shallowing out the position of the differential lever to reduce the torque, 4and hence the current to safe limits.

For stopping the vehicle in a minimum time, both the quadrant levers, 370, 370a 'may be pulled backwardly through neutral, which reverses the direction of applied torque, and into the reverse range with the reverse torque opposing the momentum of the vehicle. Under such conditions, also, the reverse torque is limited by the yielding of the actuators 371, 37M to a predetermined safe value.

One of t-he features of the present control system is that the force exerted by the torque reaction actuators 371, 371e varies in accordance with the setting of the quadrant lever. However, if desired and without departing from the present invention the actuators 371, 371a may be supplied through `a separate regulator and associated reversing and pilot valve as set forth in the system described in FIG. 10. Under such circumstances the actuators perform a protective function with the protection level being set by the adjustment of the regulator.

It will be appreciated that heavy vehicles, particularly vehicles employed for earth moving purposes and of the conventional steered or skid-steered type must operate on construction jobs which are often quite remote from service facilities. Consequently, it is noteworthy that the extreme flexibility provided by the present control and the inherent protective features have been achieved Without recourse to complicated circuitry or fragile electronic components. 0n the contrary, the entire arrangement is so simple and straightforward that service can be taken care of right in the field by personnel having no more than a rudimentary knowledge of electricity and mechanics. Thus, there is no necessity, in the event of breakdown, for waiting until an electronics technician or engineer can make the necessary correction. Nor are there any adjustments to be made by the machine operator, and which may be thrown out of kilter by one who is careless or lacks understanding. Expensive down time is minimized and machines employing the presently disclosed drive sys-tems may be sent to remote locations without fear of service problems. Nor, at the other extreme, are brute force control components required. All of the control components may be relatively small and inexpensive because of the light, short strokes which sulhce to cover the control range. Relatively light reversing contactors may be used since switching only takes place when `the stators are alined in the null condition when the current is, therefore, zero. And, as previously stated, dynamic braking suffices to perform all the necessary braking and steering functions so that the high initial expense and high maintenance cost of brakes are eliminated.

The A.C. motors may be constructed using conventional motor manufacturing techniques and the cost, per horsepower, is not substantially greater than that of comparable motors not having the present control features.

Nor is the present control system limited to special or difficult motor lcontrol applications. On the contrary, it may be employed wherever variable speed or torque are required, even for those applications where conventional, constant speed induction motors have suiced as, for example, in the driving of fans or blowers.

The versatility of the present motor control system in accommodating different types of loads is brought out by the speed torque diagram shown in FIG. 15. This ligure shows a family of speed torque curves P1-P6 corresponding to successive positions of the control member, i.e., successive amounts of offset of the stators from the null position and with assu-med resistance in .the rotor circuit to provide good starting torque. In the case of a load a downwardly concave torque characteristic as shown by the load curve L1, it will be understood in the art that operation will take place at .the conditions of equilibrium defined by the successive intersections between the load curve and the family of motor curves. Thus, a wide range of speed, here indicated by the values S1-S6 inclusive, is available. The present motor is not limited to any particular load characteristic, and it will be apparent that a load curve L2 which is concave upwardly (as in the case of a fan load) also makes a series of intersections with the curves in the motor family to define successive points of operating speed. Moreover, the motor may be readily adapted for accommodation to loads which are widely different from those shown at L1, L2 in FIG. l5. in fact, the present motor and control system may be adapted to any desired load simply by plotting the load curve and adjusting the motor parameters to produce a family of motor curves which intersect the load curve at successive points and at a substantial angle. One parameter which .is readily changed is that of rotor resistance. Another is that of the depth of the rotor slots. It is possible to bring about a more radical change by employing motor windings which are switchable between two-pole and four-pole operation at the central part of the total speed range, a matter well within the skill of the art bearing in mind the above teachings.

In accordance with one of the aspects of the invention the rotor may be blocked and output voltage may be taken fro-m the rotor slip rings, thereby employing the structure as a variable transformer, using the same control system as that disclosed, for example, in FlGS. 1-5. Such an arrangement is disclosed in FIG. 16, which shows a machine 420 having a blocked rotor assembly 430 and stators 441, 442 mounted for rotational adjustment, just as `above, under the control of a quadrant lever or suitable adjusting member 47d. One advantage of the arrangement isthat, since the reaction torque of the two stators is self-cancelling, the full range of voltage from zero to maximum may be achieved employing light control forces. Because of the light control forces the device is well suited for automatic control of voltage using light and inexpensive control elements, for example a relay R which makes one contact or another depending upon whether the voltage is below or above a desired value and a small reversible motor M which is coupled to the control member 470 for correctively adjusting it so that the output voltage will be automatically maintained at a desired value regardless of the `amount of load.

It will be understood that while the construction and control system is primarily intended for use as an induction motor it may also be employed as an induction generator upon providing means for driving the shaft and vwith the control member being settable to determine the amount of energy which is pumped into the line.

What is claimed is:

1'. A driving arrangement for a vehicle having a pair of driving wheels and a pair of steering wheels, the combination comprising a source of alternating current, separate A.C. motors coupled to the driving wheels, each of said motors having a pair of stators arranged side by side and with substantially identical windings thereon for setting up rotating fields, each of said motors having a rotor for cooperating ywith the pair of stators, the individual stators of each motor being movable relative to one another from a null phase relation to vary the output torque characteristics, a differential linkage on each of the motors connected to the stators at its respective ends for varying the stator positioning with respect to one another and having means at its neutral point for coupling to the vehicle so that the vehicle absorbs the reaction force of the stators and so that the stators are constrained to move in equal and opposite directions from said null phase relation, and means for simultaneously adjusting the linkages to produce equal amounts of st-ator movement in opposite directions from said null relation and hence the same output torque.

2. A driving arrangement for a vehicle having a pair of driving wheels and a pair of steering wheels, the combination comprising a source of alternating current, separate A.C. motors coupled to the driving wheels each of said motors having a frame including a pair of stators arranged side by side and with substantially identical windings thereon for setting up rotating fields, each of said motors having a rotor for cooperating with the pair of stators, the individual stators of each motor both being movable relative to the motor frame relative to one another from a null phase position to vary the output torque characteristics, a differential linkage on each of the motors connected to the stators at its respective ends and having means at its neutral point for coupling to the vehicle so that the vehicle absorbs the reaction force of the stators and so that the stators are consrained to move in equal and opposite directions from said null phase relation, means for simultaneously adjusting the linkages to produce equal amounts of stator movement in opposite directions from said null position and hence the same nominal output torque, and identical yielding members interposed between the neutral points of the respective linkages and the vehicle coupling means thereby to limit the maximum torque which may be exerted by the inside one of the motors during the course of executing a turn.

3. In a vehicle having a pair of driving wheels plus articulated steering wheels, the combination comprising a source of alternating current, first and second motors individually coupled to the driving wheels, each of said motors having a pair of stators fitted with substantially identical windings to produce rotating fields, each of said motors having a rotor cooperating with the pair of stators, the stators of each pair being movable from a null position, said motors both having a differential linkage with its ends connected to the associated pair of stators for moving said respective pairs of stators in opposite and equal directions from said null position, means for positioning the differential linkages so that the motors have substantially the same speed torque characteristics, and identical fluid actuators connected between the neutral points of the two differential linkages and the vehicle and having means for supplying them with liuid at the same pressure, regulator means connected to said actuators and having a predetermined yield point so that when one of the driving wheels tends to be blocked in the making of a turn the resulting tendency for buildup of torque therein eyond the predetermined value produces retreating movement at the associated actuator for corrective adjustment of the associated differential linkage and so that the torque is limited to the predetermined value,

4. In a skid-turning and driving arrangement for a vehicle having fixed driving means on each side, the cornbination comprising a source of alternating current, separate A.-C. driving motors coupled to the respective driving means, each of said motors having a pair of stators arranged side by side including separate substantially identical windings for producing respective rotating fields, each of said motors having a rotor cooperating with a pair of stators, the stators in each of the motors being relatively rotatable from a null phase position, separate means for relatively adjusting the phase positioning of the stators of the two motors by equal amounts in opposite directions from said null phase position thereby to produce variations in torque for driving and skid steering in one direction or the other.

5. In an alternating current system for driving and skid-steering a vehicle having fixed driving wheels on the sides thereof, the combination comprising a source of alternating current, A.-C. driving motors coupled to the respective driving means, each of said motors having a frame and a pair of stators arranged side by side therein and connected to the source of alternating current to produce separate rotating fields, each of said motors further having a rotor cooperating with the pair of stators, the stators comprising a pair both being movable with respect to the frame from a null position to vary the torque output characteristic, each of` said motors having a differential linkage the ends of which are connected to the respective stators for varying the stator relative positions. And the neutral point of which is coupled to the motor frame for absorbing the reactive torque of said stators, separate control means for adjusting the positions ofthe stators through the respective differential linkages thereby to increase the torque and speed of one of the motors with respect to the other to achieve a skid turn.

l 6. In an alternatingv current system for driving and skid-steering a vehicle having a frame and fixed driving means on the sides thereof, the combination comprising a source of alternating current, A.C. driving motors coupled to the respective driving means, each of said motors having a pair of stators arranged side by side and connected to the source of alternating current to produce separate rotating elds, each of said motors further having a rotor cooperating with the pair of stators, the stators comprising a pair both being relatively movable with respect to the frame from a null position to vary the torque output characteristic, each of said motors having a differential linkage the ends of lwhich are connected to the respective stators for varying the stator-relative positions and the neutral point of which is coupled to the vehicle for absorbing the reactive torque of said stators, separate control means for adjusting the positions of the stators through the respective differential' linkages thereby to increase the torque and speed of one of the motors with respect to the other toy achieve a skid turn, and means interposed between the neutral point of the differential linkage of each of the motors and the vehicle frame for yielding inthe face of the reactive torque and for thereby limiting the torque output of the respective motor to a predetermined maximum.

7,. In an electrical driving and skid-steering system for a vehicle having fixed driving means on the sides thereof, the combination comprising a source of alternating current, rst and second motors connected to the respective driving means, each of said motors having a frame mounting a pair of stators arranged side by side and connected to the :source Iof alternating current to provide adjacent rotating fields, each of said motors having a rotor cooperating with both of -said rotating fields, the stators in each of said motors being relatively rotatable with respect to one another from ak null relation to vary the output torque characteristics, each of said motors having a differential linkage connected to the associated stators at its ends for varying the stator relative positioning and having its neutral point coupled to the motor frame for absorbing the reaction torque of the stators, left and right hand manual controls for changing the relative positions of the stators through theirrespective differential linkages, each of said motors having a reversing switch interposed between the motor and the source of alternating current and means for -automatically producing switchf over of said switch from forward to reverse operation as the stators pass through their null relation so that torque is progressively applied in the forward and reverse direction by progressive forwardl and reverse movement of the manual controls.

8. In a skid-steering system for a vehicle having fixed -left and right hand driving means, the combination cornprising an A.C. source, left and right hand driving motors coupled to the driving means, respectively, each of said driving members. having a frame mounting a pair of stators connected to the A.C. source, each of said driving motors having a rotor cooperating with the pair of stators, the stators comprising a pair being relatively movable with respect to the motor frame from a null position in order to vary the torque characteristic, each of said motors having a differential linkage which is connected at its ends to the respective stators for varying the relative stator positioning and-Which is coupled at the neutral point with respect to the vehicle for absorption of the stator reactive torque, left and right hand pressure regulators each having a control member, position servos connected to the regulators for positioning the stators through respective differential linkages, and pressure actuators interposed between the differential linkages and the motor frame for yielding upon exceeding a predetermined motor torque.

9. In a skid-steeriing system for a vehicle having fixed left and right hand driving means, the -combination cornprising an A.-C. power source, left and right hand driving motors coupled to the driving means, respectively, each of said driving motors having a frame mounting a pair of stators connected to the A.-C. source, each of said driving motors having a rotor cooperating with the pair of stators, the stators of each motor being relatively movable with respect to the motor frame from a null position in order to vary the output torque characteristic, each of said motors havinga differential linkage which is connected at its ends to the respective stators for varying the stator relative positions and which is coupled at its neutral point with respect to the motor frame for absorption of the stator reactive torque, left and right hand pressure regulators each having a control member movable in forward and reverse directions from neutral for buildup of pressure in opposite directions, position servos connected to the respective regulators for positioning stators through the respective differential linkages and coupled to the pressure regulators for limiting the output torque of the motors in accordance with the setting of the left and rig-ht hand control members.

1t). In a skid-steering system for a vehicle having fixed left and right hand driving means, the combination com.- prising a source of alternating current, left and right hand A.-C. driving motors coupled to the respective driving means, each of said driving motors having a pair of stators -arranged side by side and electrically connected to the source of alternating current, each of said motors having a rotor cooperating with the pair of stators, the stators of each pair being relatively movable from a null position in each direction for progressive increase in output torque, each of said motors yhaving a differential linkage for coupling together the pair of stators for varying the stator relative positioning with the neutraly point thereof being connected to the frame for absorbing the stator reaction torque, left and right hand control members coupled to the respective differential linkages for moving the same in both directions from the null position, left and right hand reversing switches interposed between the pairs of stators and the source of alternating current and having means for switching as the stators pass through their relative null positions so that continuous movement of each control member from the forward to the reverse positions is effective to cause (a) progressive decrease in forward torque (b) reversal of the motor windings when the torque is reduced to Zero and (c) progressive increase in reverse torque for braking and reversal.

11. In a skid-steering system for a vehicle having a fixed left and right hand driving means, the combination comprising a source of alternating current, left and right -hand A.C. driving motors coupled to the respective driving means, each of said driving motors having a pair of stators arranged side by side and electrically connected to the source of alternating current, each of said motors having a rotor cooperating with the pair of stators, the stators of each pair being relatively movable from a null position for progressive incre-ase in output torque, each of said motors having a differential linkage for coupling together the pair of stators for varying the relative positioning of the same and for absorbing the stator reaction torque, left and right hand control members coupled to the respective differential linkages for mov ing the same together with said respective stators from the null position, left and right hand reversing switches interposed between the pairs of stators and the source of alternating current and having means foreffecting switching when the stators pass through in their relative null positions so that continuous movement of e-ach control member from the forward to the reverse positions is effective to cause (a) progressive decrease in forwaid torque (b) reversal of the motor windings when the torque is reduced to zero and (c) progressive increase in 727,662 5/ 1903 Meuschel 318-234 X reverse torque for braking and reversal. 1,863,504 6/ 1932 Schmid 180-6.5 2,600,523 6/ 1952 Ellis B18-243 References Cited by the Examiner 644,853 3 /1900 Fahl 130 6'5 X I. A. PEKAR, Assistant Examiner. 

5. IN AN ALTERNATING CURRENT SYSTEM FOR DRIVING AND SKID-STEERING A VEHICLE HAVING FIXED DRIVING WHEELS ON THE SIDES THEREOF, THE COMBINATION COMPRISING A SOURCE OF ALTENATING CURRENT, A.-C DRIVING MOTORS COUPLED TO THE RESPECTIVE DRIVING MEANS, EACH OF SAID MOTORS HAVING A FRAME AND A PAIR OF STATORS ARRANGED SIDE BY SIDE THEREIN AND CONNECTED TO THE SOURCE OF ALTERNATING CURRENT TO PRODUCE SEPARATE ROTATING FIELDS, EACH OF SAID MOTORS FURTHER HAVING A ROTOR COOPERATING WITH THE PAIR OF STATORS, THE STATORS COMPRISING A PAIR BOTH BEING MOVABLE WITH RESPECT TO THE FRAME FROM A NULL POSITION TO VARY THE TORQUE OUTPUT CHARACTERISTIC, EACH OF SAID MOTORS HAVING A DIFFERENTIAL LINKAGE THE ENDS OF WHICH ARE CONNECTED TO THE RESPECTIVE STATORS FOR VARYING THE STATOR RELATIVE 